More than 4.5 million children in the UK are growing up in areas with toxic levels of air pollution, the UN children’s organisation Unicef has warned.

Tests suggesting that children walking along busy roads are exposed to a third more air pollution than adults, as their shorter height places them close to passing car exhausts, were also released on Thursday.

Amy Gibbs, at Unicef UK, said: “The findings force us to face a shocking reality about the acute impact on children’s health. Worryingly, one third of our children could be filling their lungs with toxic air that puts them at risk of serious, long-term health conditions.

“It’s unacceptable that the most vulnerable members of society, who contribute the least to air pollution, are the ones suffering most from its effects,” she said. “The government must accept this is a children’s health crisis and offer targeted action and funding to reduce their exposure.”

The tests on children’s exposure next to busy roads are relevant to the millions of children walk to school each day, with experts are advising that where practical parents choose quieter routes, away from traffic, as this can cut pollution exposure by almost two-thirds. Other scientists have suggested parents use covers on their prams and buggies during the school run to protect their infants from air pollution. Half of all children walk to school, but being driven to school by car instead can actually result in greater pollution exposure for those inside the vehicle, previous research has shown.

Prof Jonathan Grigg, at Queen Mary University of London, said: “My research has shown that exposure of young children to higher amounts of air pollution from traffic has a major impact on their lungs. Although parents can reduce this impact by walking on less polluted roads, the UK government must take further steps to reduce toxic emissions on all roads.”

The environment secretary, Michael Gove, said the school run tests were troubling: “This a further demonstration of why we need to take strong action now to improve air quality.” He said the government was acting, but added: “By taking simple steps, like leaving the car at home for the school run, we can work together to reduce air pollution and protect our health.”

Lack of funding for local authorities to tackle air pollution is a key issue, the select committee MPs said. “The car industry is partly responsible for our toxic streets, and seeing the government resist calls for an industry-financed Clean Air Fund is incomprehensible,” said Neil Parish MP, chair of the environment committee.

The school run tests analysed particulate pollution on different routes taken by primary and nursery schoolchildren in London, Manchester and Leeds. Small measuring devices were carried by each child and adult, with one measurement taken on each route in each city. In Manchester, a test found that the upper deck of a bus was much less polluted than the lower deck.

The tests were commissioned by Global Action Plan for Clean Air day, which falls on Thursday and is supported by the government and 180 organisations. The organisers are calling on people to leave their car at home where possible. When streets were closed to traffic for the 2018 London marathon, pollution levels dropped by 89%.

Mala Kapoor, who took part in the tests in Leeds with her daughter Ariyan, said: “I was shocked to hear that children are more exposed than adults to air pollution from exhaust fumes. When going out I do try to take more back routes – it might take me a couple of minutes longer but if it reduces Ariyan’s exposure to air pollution, then it’s worth it.”